Very Large Scale Integration (VLSI), Ultra Large Scale Integration (ULSI), and higher-density approaches to integrated circuit fabrication require reduced parasitic capacitances. One approach to reducing these capacitances is the use of silicon-on-insulator (SOI) techniques.
In SOI fabrication, layers of device-quality silicon are formed upon insulating islands, as shown in FIG. 1. In one method of SOI fabrication, silicon is grown upon sapphire (the INSULATOR in FIG. 1) in an epitaxial growth step.
In another method, shown in FIGS. 2A-2D, oxygen (or nitrogen) is implanted through silicon, to form a buried layer of silicon dioxide (or silicon nitride). After implantation, an epitaxial layer 3 may be added to the silicon located above the implant, in order to provide a thicker silicon layer. The silicon layer 6 provides the device-quality silicon.
SOI technology is not without drawbacks. Epitaxial approaches have several disadvantages.
1. Epitaxial growth is expensive. PA1 2. The epitaxially grown silicon is not always sufficiently defect-free. PA1 3. Adhesion problems can result between the epitaxial layer and the substrate. PA1 4. Stresses can occur at the silicon-insulator interface, because of (a) different thermal coefficients of expansion, and (b) mismatch between the inter-atomic spacing of the silicon, compared with that of the insulator. PA1 1. The implantation approach requires implantation equipment, which is expensive. PA1 2. During implantation, the implanted species disrupts the crystalline structure of the silicon. An annealing step is required to restore proper crystallinity of the silicon. PA1 3. Practical implantation energies cannot supply the required implant depth for most implant species. Further, a layer 3 in FIG. 2C is frequently obtained, in which no substantial implantation occurred. Sometimes, this layer 3 may be too thin, requiring that the thickness of layer 3 be increased. The increase in thickness is generally accomplished by epitaxial techniques, which require added processing steps, shown in FIG. 2D. PA1 4. There is a limit to the amount of implanted species which can be implanted. Thus, the doping concentration, and thus the conductivity, of the implanted layer 6 in FIG. 2D is limited.
Implantation approaches also face disadvantages.